Visible laser projection systems are now widely used by industry to project a laser outline, laser image or laser template on a target surface or work surface for assembling large two or three-dimensional structures or assemblies, such as prefabricated roof trusses and aerospace composites. By precisely characterizing the laser projection and establishing the exact relative position of the laser projector to the assembled structure, composite or target work piece, the laser projector system is capable of accurately producing a laser image or template at precisely known coordinates on a target surface or work surface which may be planar or curvilinear. For example, U.S. Pat. No. 5,646,859 assigned in part to the assignee of this application discloses a method and apparatus for defining a laser template for assembling the components of a structure, such as a prefabricated roof truss. The method and apparatus disclosed in this patent includes a laser projector or a plurality of laser projectors mounted above a work surface, a plurality of sensors or laser targets fixed at predetermined locations on or adjacent the work surface, a computer and a sensor on the laser projector. The laser projector periodically or continuously scans the laser targets, such as laser targets having a retro-reflective surface, and a reflected light from the laser targets to the sensor of the laser projector determines the precise projection angle associated with each of the laser target datum. Using a series of mathematical algorithms, the precise position and orientation of the laser projector relative to the work surface is then calculated by a computer associated with the laser projector. The spatial information in conjunction with a known display list or data stored in the computer allows the laser projector to generate accurate laser templates on the work surface.
In the assembly of a prefabricated roof truss assembly, for example, the computer may control the laser projector or projectors to project an image or laser outline of the components of the truss assembly in the assembled position on the work surface including the chords, webs, nail plates, etc. and the components may then assembled on the laser outline and secured in place as by pressing or rolling the assembled components, thereby driving the nail plates into the chords and webs. Where a number of identical truss assemblies are made in mass production applications, jigs movable on the work table may be used to align the components of the assembly and the jigs may be located on the work surface using the laser projector as disclosed, for example, in U.S. Pat. No. 6,170,163 assigned to the assignee of this application. Improvements in assembly tables have also been made, such as the “Slotted-Top Tables” available from MiTek Industries, Inc. of Chesterfield, Mo., which include a plurality of linear elongated ejector slots in the table having pneumatically operated “pop-up” ejectors which lift the assembled truss from the assembly table. The use of MiTek's Slotted-Top Table in combination with the RoofTracker® laser projection system available from the assignee of this application provides a very efficient apparatus for assembly of roof trusses.
However, it would be desirable to automatically track the number of trusses assembled, for example, the number of trusses of each design or specification assembled, and the time of assembly of each truss without requiring wires or cables between the assembly table and the computer.